Acquired defense mechanisms against the intestinal dwelling nematode, Trichinella spiralis, will be analyzed in rats and mice with a view to defining the mechanism of rapid expulsion and genetic factors that influence the development and expression of host resistance to infection. Two stimuli are required for the full expression of rapid expulsion in rats. One of these stimuli, provided by adult worms, seems to lack immunological specificity; however, more rigourous evidence is needed to substantiate this notion. The possibility that the above stimulus is related to irritation (inflammation) of the intestinal wall will be examined in experiments using various inhibitors of inflammation, such as salicylates, adrenal corticosteroids, indomethacin, and the factor in cobra venom which cleaves C3. Genetic studies of the rapid expulsion response will be pursued in mice. It has been shown that the capacity to express rapid expulsion is inherited as a dominant characteristic. Back cross experiments are planned to test the proposition that responsiveness is transferred by a gene that lies within or is closely linked to the H-2 complex. If this should prove to be the case, the gene would be mapped in experiments using congenic mice and recombinant strains.